A transparent conductive film, because of having high conductivity and high transmittance in a visible light region, has been utilized in an electrode or the like, for a solar cell or a liquid crystal display element, and other various light receiving elements, as well as a heat ray reflection film for an automotive window or construction use, an antistatic film, and a transparent heat generator for various anti-fogging for a refrigerator showcase and the like.
As a well known practical transparent conductive film, there has been included a thin film of tin oxide (SnO2)-type, zinc oxide (ZnO)-type, indium oxide (In2O3)-type. As the tin oxide-type, the one containing antimony as a dopant (ATO), or the one containing fluorine as a dopant (FTO) has been utilized, and as the zinc oxide-type, the one containing aluminum as a dopant (AZO), or the one containing gallium as a dopant (GZO) has been utilized. However, the transparent conductive film most widely used industrially is the indium oxide-type. Among them, indium oxide containing tin as a dopant is called an ITO (Indium-Tin-Oxide) film, and has been utilized widely, because, in particular, a film with low resistance can be obtained easily.
The transparent conductive film with low resistance is suitably used widely in a surface element or a touch panel or the like, of such as for a solar cell, a liquid crystal, an organic electroluminescence and an inorganic electroluminescence. As a production method for these transparent conductive films, a sputtering method or an ion plating method has been used often. This sputtering method is an effective method in film-formation of a material with low vapor pressure, or when control of precise film thickness is required, and because of very simple and easy operation thereof, it has been widely used industrially.
In a sputtering method, a target for sputtering is used as a raw material of a thin film. The target is a solid containing a metal element constituting the thin film to be formed, and a sintered body such as a metal, a metal oxide, a metal nitride, a metal carbide, or, in certain cases, a single crystal is used. In this method, in general, after making high vacuum once with a vacuuming apparatus, rare gas (argon or the like) is introduced, and under a gas pressure of equal to or lower than about 10 Pa, a substrate is set as an anode and a target is set as a cathode to generate glow discharge between them and generate argon plasma, and argon cations in the plasma are collided with the target of the cathode, and particles of the target component flicked thereby are deposited on the substrate to form a film.
A sputtering method is classified by a generation method of argon plasma, and a method using high frequency plasma is called a high frequency sputtering method, and a method using direct-current plasma is called a direct-current sputtering method.
In general, a direct-current sputtering method has been utilized industrially in a wide range, because it provides higher film-formation rate and lower cost of power source facility and simpler film-formation operation, as compared with the high frequency sputtering method. However, the direct-current sputtering method has a disadvantage of requiring use of a conductive target, as compared with the high frequency sputtering method, which can provide film-formation even by using an insulating target.
Film-formation rate of a sputtering has close relation to chemical bond of a target substance. Because a sputtering is a phenomenon that argon cations having a kinetic energy are collided to the target surface, and a substance of a target surface is flicked by receiving energy, the weaker inter-ionic bond or inter-atomic bond of the target substance increases the more probability of jumping out by sputtering.
In film-formation of a transparent conductive film of an oxide such as ITO by using a sputtering method, there are a method for film-formation of an oxide film by a reactive sputtering method in mixed gas of argon and oxygen, by using an alloy target (an In—Sn alloy in the case of the ITO film) of metals constituting the film, and a method for film-formation of an oxide film by a reactive sputtering method for performing a sputtering in mixed gas of argon and oxygen, by using an oxide sintered body target (an In—Sn—O sintered body in the case of the ITO film) composed of an oxide of metal constituting the film.
Among these, in a method for using the alloy target, relatively high amount of oxygen gas is supplied during sputtering, however, because dependence of film-formation rate or film characteristics (specific resistance, transmittance) on amount of oxygen gas to be introduced during film-formation is extremely high, it is difficult to produce stably a transparent conductive film having a constant film thickness or characteristics.
On the other hand, a method for using the metal oxide target supplies a part of oxygen supplied to the film from the target by sputtering, and thus residual deficient oxygen amount is supplied as oxygen gas, and dependence of characteristics (specific resistance, transmittance or the like) of the film on film-formation rate or oxygen gas amount to be introduced during film-formation is smaller than the case of using the alloy target, and because the transparent conductive film having constant film thickness and characteristics can be produced stably, a method for using the oxide target has been adopted industrially.
From such a background, in the case of mass production of the transparent conductive film by film-formation using the sputtering method, the direct-current sputtering method using the metal oxide target has been mainly adopted. Here, in consideration of productivity or production cost, characteristics of the oxide target during direct-current sputtering become important. That is, such an oxide target is useful that provides higher film-formation rate in the case of charging the same power. Still more, because film-formation rate becomes the higher, when the higher direct-current sputtering is charged, such an oxide target becomes useful industrially that is capable of film-forming stably without generation of target cracking or abnormal discharge caused by arcing due to nodule generation, even when high direct-current power is charged.
Here, the nodule means a black precipitate (protruded substance) generating at an erosion part of the target surface (meaning a site of the target being sputtered), excluding a very small part at the deepest part of erosion, when sputtering of the target proceeds. In general, the nodule is said not to be a deposition of a foreign flying substance or a reaction product at the surface, but a digging residue by sputtering. The nodule causes abnormal discharge such as arcing, and it has been known that arcing is suppressed by reducing the nodule (refer to NON-PATENT LITERATURE 1). Therefore, for performing stable film-formation, use of such an oxide target is necessary that does not generate the nodule, that is, a digging residue by sputtering.
On the other hand, the ion plating method is a method for evaporating a metal or a metal oxide by resistance heating or electron-beam heating, under a pressure of about 10−3 to 10−2 Pa, and still more activating the evaporated substance using plasma and reaction gas (oxygen) to deposit it on a substrate. Also as for the target for ion plating (it may also be called a tablet of pellet) to be used in forming the transparent conductive film, similarly as in the target for sputtering, use of an oxide tablet enables to more stably produce a transparent conductive film having constant film thickness and constant characteristics. The oxide tablet is required to evaporate uniformly, and it is preferable that a substance having stable chemical bond and difficult to be evaporated is not present together with a substance which is present as a main phase and easily evaporated.
As described above, it can be said that in order to form the transparent conductive film of an oxide such as ITO by the direct-current sputtering method or the ion plating method, such an oxide target is important that enables stable film-formation without generation of abnormal discharge caused by arcing or the like due to nodule generation.
By the way, many of the transparent conductive films such as ITO film, formed by the above process, are n-type degenerated semiconductors, and largely contribute to enhance conductivity of electrons of carriers. Therefore, conventionally, in order to make low resistance of the ITO film, carrier electron concentration has been made to increase as high as possible.
The ITO film has been known to have a crystallization temperature of generally about 190 to 200° C., and bordering on this temperature, an amorphous film or a crystalline film is formed. For example, in the case of film-formation by a sputtering method while maintaining the substrate at room temperature, the amorphous film is obtained, because thermal energy required in crystallization cannot be given. On the other hand, in the case where a substrate temperature is equal to or higher than crystallization temperature, for example, about 300° C., the crystalline film is formed.
In the amorphous film and the crystalline film of ITO, generation mechanism of carrier electrons is different. In general, in the case of the amorphous film, nearly all of the carrier electrons are generated by oxygen deficiency. On the other hand, in the case of the crystalline film, generation of the carrier electrons is expected by not only oxygen deficiency but also tin doping effect.
Indium oxide takes a crystal structure called bixbyite of a stable cubic system crystal phase, under normal pressure or pressure lower than that. By substitution of a lattice point of tri-valent indium in the bixbyite structure with tetra-valent tin, the carrier electrons are generated. Tin is an element which is capable of increasing carrier electron concentration most, as a dopant, and it has been known that the addition of 10% by weight as converted to tin oxide is capable of making low resistance most. That is, by converting the ITO film to a crystalline film, carrier electrons are generated in a large quantity by both of oxygen deficiency and the tin dopant, and therefore it is possible to form a film showing lower electric resistance as compared with an amorphous film having only oxygen deficiency.
However, in an LED (light Emitting Diode) or a solar cell whose progress has been significant in recent years, there has emerged a case requiring characteristics which is difficult to attain by ITO. As one example thereof, in a blue LED, to enhance light extraction efficiency, high refractive index of the transparent conductive film has been necessary for blue light at the vicinity of a wavelength of 460 nm. As a light emitting layer of the blue LED, a gallium nitride layer is used. As an optical characteristics of the gallium nitride layer, refractive index as high as about 2.4 is included. In order to enhance efficiency of light extraction from the light emitting layer, it is necessary to enhance consistency of refractive indexes of the transparent conductive film and the gallium nitride layer, and the transparent conductive film is required to have a refractive index of as near as 2.4. Refractive index is a value specific to a substance, and generally known refractive index of indium oxide is as low as 1.9 to 2.0. In addition, the transparent conductive film is required to have low surface resistance. It is because current diffusion is not sufficient in a film surface direction, as electrical characteristics of the gallium nitride layer. However, when it is tried to decrease electric resistance by increasing carrier electron concentration, refractive index of the indium oxide-type transparent conductive film becomes lowered further than 1.9 to 2.0 to 1.8 to 1.9. As described above, because the ITO film is a material having significantly increased carrier electron concentration owing to tin as a dopant, trying to obtain a crystalline film with such a low resistance results in decreasing refractive index, and this has been a problem to be solved.
In addition, other than refractive index or specific resistance, characteristics such as patterning property by wet etching or the like, superior than that of the ITO film, is required. Also in the above blue LED, such a process is preferable that makes low resistance by performing patterning by wet etching using a weak acid on the amorphous transparent conductive film formed at low temperature, and then by heat treatment under non-oxidative atmosphere to crystallize the amorphous transparent conductive film. By using this process, it is possible to form a transparent electrode having highly fine patterning.
As other application examples of the transparent conductive film, there is a solar cell. In the case of using it as a surface electrode of a solar cell, when the transparent conductive film has high transmittance of not only visible light but also infrared light, solar light can be taken in efficiently. The ITO film is capable of decreasing specific resistance, however, because of high carrier electron concentration, there was a problem of high reflectance or absorption of infrared light, and thus decreasing transmittance.
In addition, in the case of using it as a part of a rear surface electrode, there may be the case of using a transparent conductive film having enhanced refractive index, for performing adjustment of refractive index of the whole module, aiming at enhancing incorporation efficiency of solar light, however, also in this case, the ITO film was insufficient because of the same reason as in a blue LED application. However, in a solar cell application, it is not required such high-definition patterning by wet etching using a weak acid, that is required in the blue LED.
As one method for enhancing refractive index of the indium oxide-type transparent conductive film, there is a method for adding an oxide having high refractive index.
In PATENT LITERATURE 1, there has been described a sputtering target, which is capable of efficiently forming a transparent thin film with superior moisture-proof property, on a silver-type thin film by a sputtering method, and gives little damage to the above silver-type thin film, and there has been proposed a sputtering target composed of a conductive transparent metal oxide containing an oxide of a metal element substantially not having a solid solution region with silver, wherein content ratio of the above metal substantially not having a solid solution region with silver, is 5 to 40% by atom relative to the metal element of the conductive transparent metal oxide. Specifically, it has been described that containing of at least a titanium element or a cerium element is preferable, as the metal element substantially not having a solid solution region with silver, and as a metal element similarly applicable, there has been included a zirconium element, a hafnium element, a tantalum element. In addition, there has been described that indium oxide is preferable as the conductive transparent metal oxide.
In addition, in PATENT LITERATURE 1, there has been described that because the metal oxide of the titanium element or the cerium element, which is the metal element substantially not having a solid solution region with silver, has a high refractive index of equal to or higher than 2.3, and as said high refractive index material, total content rate of the titanium element and the cerium element is 5 to 40% by atom relative to the metal element of the conductive transparent metal oxide, it is possible to increase refractive index of the transparent conductive film, formed by using this sputtering target, up to about 2.1 to 2.3.
In addition, in PATENT LITERATURE 2, there has been proposed a sputtering target of a sintered body of a mixed oxide applicable in film-forming a transparent thin film of a conductive film with a composition sandwiching the silver-type thin film. In film-formation of the transparent thin film of the conductive film with a configuration sandwiching the silver-type thin film, in order to be able to effectively perform film-formation of the transparent thin film with superior moisture-proof property, and also to obtain a sputtering target where the above silver-type thin film little receives damage in this film-formation, specifically, a sintered body of the mixed oxide having contained tin oxide and/or titanium oxide in an amount lower than mixing ratio of each substrate, to a mixed oxide having indium oxide and cerium oxide as the substrate, is used. That is, similarly as in PATENT LITERATURE 1, because cerium oxide has high refractive index, also refractive index of the mixed oxide of indium oxide and cerium oxide becomes high, accompanying with addition ratio of cerium oxide.
Still more, because in the mixed oxide of indium oxide and cerium oxide, cerium oxide does not have sufficient conductivity, conductivity of a target using a sintered body of the mixed oxide abruptly decreases accompanying with increase in mixing ratio of cerium oxide, and thus providing a target with low conductivity, which makes difficult film-formation by a direct-current sputtering method.
As described above, according to PATENT LITERATUREs 1 and 2, it is expected to increase refractive index of the transparent thin film, formed by using this sputtering target, up to about 2.1 to 2.3, because efficient formation of the transparent thin film with superior moisture-proof property is possible by a sputtering method on the silver-type thin film, or because a metal oxide of a titanium element or a cerium element has a high refractive index of equal to or higher than 2.3. However, as described above, in the case of mass production of the transparent conductive film by film-formation using a direct-current sputtering method, in view of industrial usefulness of such an oxide target that is capable of stable film-formation without generation of target cracking or abnormal discharge caused by arcing due to nodule generation, even when high direct-current power is charged, it is necessary that nodule generation causing the above arcing is suppressed, or splash in an ion plating method is suppressed, when condition to increase film-formation rate, by increasing sputtering voltage or the like, is selected, but investigation on a texture or the like of the oxide sintered body enabling it has not been performed at all.
That is, there has not been considered to the point of industrially required characteristics, relating to the oxide sintered body to obtain a target applicable to stable film-formation of the above transparent conductive film.
Still more, in PATENT LITERATUREs 1 and 2, although there has been investigated a method for production a sintered body to obtain a target, or a method for enhancing conductivity, by the simple addition of tin oxide or titanium oxide, there has not been investigated at all a method for enhancing density of a sintered body by detailed analysis and control of a texture of the oxide sintered body containing indium and cerium as oxides; or a method for avoiding arcing in film-formation using the above sputtering method, or splash in film-formation using the ion plating method. In addition, as for the case where a crystalline transparent conductive film was formed, there has not been investigated at all influence of tin oxide or titanium oxide, which is the addition element, on refractive index of the transparent conductive film.
On the other hand, in PATENT LITERATURE 3, there has been proposed an amorphous transparent conductive thin film which is extremely smooth and has a high work function, an oxide sintered body which is capable of forming stably said transparent conductive thin film, and a sputtering target using the same, and there has been described that it is desirable that said oxide sintered body contains 3% by mass to 20% by mass of cerium, 0.1% by mass to 4% by mass of tin, and 0.1% by mass to 0.6% by mass of titanium, and the remaining is substantially composed of indium and oxygen, and still more cerium, tin and titanium make a solid solution in an indium site, sintering density is equal to or higher than 7.0 g/cm3, and average crystal grain diameter is equal to or smaller than 3 μm.
Also in this PATENT LITERATURE 3, there has not been investigated at all enhancement of refractive index of the crystalline transparent conductive film formed by using said sputtering target. In particular, there has not been referred at all to influence of tin on decrease in refractive index.
Still more, as for said oxide sintered body, average particle diameter of a crystal grain of indium oxide, where cerium, tin and titanium make a solid solution in an indium site, is controlled at equal to or smaller than 3 μm, aiming at suppressing sintering crack during sputtering and nodule generation at that part, however, there has not been investigated at all a problem that cerium does not make a solid solution in indium oxide, and is present as crystal grains of cerium oxide, which becomes a starting point of a nodule.
In addition, in PATENT LITERATURE 4, there has been described a sputtering target characterized in that, in a sputtering target composed of indium oxide and cerium oxide, in the case of observing crystal peaks using X-ray diffraction, presence of peaks derived from indium oxide and cerium oxide are observed, and in performing EPMA measurement, diameter of a cerium oxide particle dispersed in indium oxide is found to be equal to or smaller than 5 μm.
This PATENT LITERATURE 4 is a sputtering target composed of indium oxide and cerium oxide, and has not investigated at all enhancement of refractive index and decrease in resistance of a crystalline transparent conductive film formed by using said sputtering target composed of indium oxide and cerium oxide, and not added with titanium or the like. In particular, there has not been referred at all to influence of tin on decreasing refractive index.
As described above, in conventional technology relating to an oxide sintered body containing indium and cerium having low specific resistance and high refractive index, sufficient investigation has not been performed on nodule suppression in sputtering film-formation, or splash prevention or the like in ion plating film-formation, which becomes important in view of mass production of a crystalline transparent conductive film, and it has been desired emergence of an oxide sintered body containing indium and cerium, which has solved these problems.